1. Field of the Invention
The invention relates to electrical cables. More particularly, the invention relates to electrical cables which exhibit low resistance, high fatigue strength, low weight, good flexibility, cool operation, minimized parasitic capacitance, and which are renitent to the adverse affects of aging and corrosion.
2. State of the Art
My prior application which is referenced above describes the general techniques known in the art for making electrical cables from helically twisted filaments and proposes methods of twisting and drawing wire cables for enhancing the conductivity, flexibility and tensile strength of the cables. In addition to low resistance, flexibility and tensile strength, other characteristics of cables may be important depending on the application in which the cable is used. For example, the ability of a cable to remain cool during operation is often an important consideration. For cables used outdoors for power transmission, renitence to corrosion and low weight of the cable are important considerations. For cables which are subjected to repeated flexion, good flexibility as well as high fatigue strength are important. In cables which are used as leads for semiconductors and other electronic components, parasitic capacitance is an important consideration.
Among the many factors which affect the resistance of an electrical conductor is its temperature. As the temperature of the conductor rises, so does its resistance. Moreover, as the resistance of a conductor is increased, current passing through the conductor will further heat the conductor. Known techniques for cooling electrical conductors are complex and expensive.
The usual method for preventing or minimizing the effects of corrosion on an electrical conductor is to cover it with insulation. However, in many applications, such as power transmission cables, insulation can significantly add to the cost of the cable. Most "high tension" power transmission cables are not covered with insulation.
In most cables, their weight is dictated by the choice of materials and the dimensions of the cable. Often, attempts to reduce the weight of the cable results in either increased cost of the materials used to fabricate the cable or an increase in the resistivity of the cable.
Fatigue strength is an important characteristic of electrical conductors which are subjected to flexion such as overhead power cables, flexible power cords, communications cables, and wires used in hand held or portable equipment. The usual method of increasing fatigue strength is to utilize a stranded conductor rather than a solid conductor. As explained in my earlier application, a stranded conductor with the same conductivity of a solid conductor will have a greater cross sectional diameter than the solid conductor. The larger stranded conductor also requires more insulation and has increased parasitic inductance. In addition, as the overall diameter of the insulated stranded conductor increases, so does its stiffness. In general, the stiffness of cylindrical bending beams tends to increase exponentially (to the fourth power) as the diameter is increased. Also, as the number of strands is increased, the ratio of surface area to cross sectional area increases. This makes the cable more vulnerable to the damaging effects of corrosion which significantly decrease the conductivity of the cable as it ages in service.
Parasitic capacitance is a characteristic of electrical conductors which is very important in some applications such as leads for semiconductors and other electrical components. The most common technique for minimizing parasitic capacitance is to make the conductive leads as small as possible and to separate them as much as possible. However, making the leads smaller (in diameter) increases their resistance; and separating the leads from each other results in a larger package size for the component.
As mentioned above, stranded conductors have increased parasitic inductance as compared to solid conductors of the same diameter. This is mostly the result of the individual strands being helically wound rather than being arranged parallel to the axis of the conductor. To the extent that the individual strands are not in perfect electrical contact with each other along their entire length, they tend to behave as individual helical conductors, i.e. as coils. Stranded cables with strands having circular cross section always exhibit imperfect electrical contact among the strands because they contact each other only along a single line. Cables having preformed strands (e.g. "trapezoidal wire") also exhibit imperfect electrical contact among the strands because they contact each other only along portions of their surfaces. An attendant and related problem occurs with customary stranded cables as they age in the presence of air, moisture and other corrosive agents. The buildup of surface corrosion on the strands further reduces the electrical contact among the strands further increasing parasitic inductance.